The carbon isotopic composition of catalytic gas: a comparative analysis with natural gas

Abstract

The idea that natural gas is the thermal product of organic decomposition has persisted for over half a century. Crude oil is thought to be an important source of gas, cracking to wet gas above 150°C, and dry gas above 200°C. But there is little evidence to support this view. For example, crude oil is proving to be more stable than previously thought and projected to remain intact over geologic time at typical reservoir temperatures. Moreover, when oil does crack, the products do not resemble natural gas. Oil to gas could be catalytic, however, promoted by the transition metals in carbonaceous sediments. This would explain the low temperatures at which natural gas forms, and the high amounts of methane. This idea gained support recently when the natural progression of oil to dry gas was duplicated in the laboratory catalytically. We report here the isotopic composition of catalytic gas generated from crude oil and pure hydrocarbons between 150 and 200°C. δ 13C for C 1 through C 5 was linear with 1/ n ( n = carbon number) in accordance with theory and typically seen in natural gases. Over extended reaction, isobutane and isopentane remained lighter than their respective normal isomers and the isotopic differentials were constant as all isomers became heavier over time. Catalytic methane, initially −51.87‰ (oil = −22.5‰), progressed to a final composition of −26.94‰, similar to the maturity trend seen in natural gases: −50‰ to −20‰. Catalytic gas is thus identical to natural gas in molecular and isotopic composition adding further support to the view that catalysis by transition metals may be a significant source of natural gas.